1. Field of the Invention
The present invention relates to a method of manufacturing a thin film transistor and an active matrix substrate using the thin film transistor as a switching element.
2. Description of the Related Arts
A thin film transistor (hereinafter referred to as a TFT) is utilized for the switching element of a pixel electrode of an active matrix type liquid crystal display device. As the demand for a high-definition liquid crystal display grows, the semiconductor layer of the TFT is required to be formed of polycrystalline silicon instead of amorphous silicon.
If the semiconductor layer of a TFT is formed of polycrystalline silicon, it is possible to manufacture the TFT such that it has high mobility and a large on-current, and hence not only a pixel matrix circuit but also a driver circuit can be integrally formed on the same substrate. However, in the TFT using the polycrystalline silicon, a current leaking from a drain in an off state (that is, off current) is large, and hence, if it is used as the switching element of a matrix circuit, it can not hold the electric potential of the pixel electrode. Therefore, it has been a big problem to reduce the off current of the TFT.
In order to solve this problem, an attempt has been made to relieve voltage concentrated on the drain and to reduce the off current by making a TFT having an offset structure or a light doped drain structure (hereinafter referred to as an LDD structure). A method of manufacturing the TFT having the LDD structure will briefly be described by the use of FIG. 9.
An underlayer film 11 made of a silicon oxide film is formed on a glass substrate 10. An amorphous silicon film is formed on the underlayer film 11 and is polycrystallized by applying an excimer laser thereto. The polycrystallized silicon film is patterned in a shape of island to form a semiconductor layer 12. A gate insulating film 13 made of silicon oxide is formed such that it covers the semiconductor layer 12. A metal film made of aluminum, tantalum, or the like is formed on the gate insulating film 13. A photoresist mask 14 is formed and the metal film is patterned in a predetermined shape by using the photoresist mask 14 to form a gate electrode 15 (see FIG. 9 (A)).
The photoresist mask 14 is removed and then impurities to be donors or acceptors are added to the semiconductor layer 12 by ion doping or by ion implantation by using the gate electrode 15 as a doping mask, whereby impurity regions 16, 17 are formed in the semiconductor layer 12 in a self-alignment manner (see FIG. 9 (B)).
A photoresist mask 18 is formed which is wider in the direction of length of channel than the gate electrode 15. The length of a low-concentration impurity region is determined by the shape of the photoresist pattern 18 (see FIG. (C)).
Impurities to be donors or acceptors are added to the semiconductor layer 12 by ion doping or by ion implantation by using the photoresist pattern 18 as a doping mask, whereby a source region 21, a drain region 22, and low-concentration impurity regions 24, 25 are formed in the semiconductor layer 12 (see FIG. 9 (D)).
The photoresist pattern 18 is removed and then the impurities added to the semiconductor layer 12 are activated by applying laser light to the substrate or by heating the substrate. An interlayer insulating film 27 comprising silicon oxide film is formed. Contact holes, that lead to the source region 21, the drain region 22, or the terminal part (not shown) of the gate electrode 15, are made in the interlayer insulating film 27. A metal film made of titanium or the like is formed and is patterned to form a source electrode 28, a drain electrode 29 and the lead wiring (not shown) of the gate electrode 15 (see FIG. 9 (E)).
In a conventional manufacturing method shown in FIG. 9, the photoresist pattern 18 is used as a doping mask so as to make an LDD structure. Therefore, in order to form the low-concentration impurity region with high accuracy, a photolithography mask is required to be aligned with high accuracy, but there is a problem that as an element becomes finer and a liquid crystal panel becomes larger in area, an alignment accuracy becomes lower.
Therefore, in order to solve the above problem, the present applicant discloses a technology for manufacturing a TFT having an LDD structure in a self-alignment manner in Japanese Patent No. 2759415. In the above Japanese Patent, aluminum is used as a gate electrode material and the LDD structure is formed in a self-alignment manner by using an anodic oxide (alumina) by an oxalic acid and an anodic oxide (alumina) by a tartaric acid as the doping masks.
In the above Japanese Patent, a photoresist is not used as the doping mask and hence the length of the low-concentration impurities region can be controlled with high accuracy, but there is a drawback that the gate electrode material is limited to aluminum. Also, there is another problem that the process temperature is limited to about 400° C. after an aluminum wiring is formed and that aluminum atoms are diffused into a gate insulating film to easily make a short circuit between a gate wiring and a channel, thereby reducing reliability.
Further, in an anodic oxidation process, each gate electrode/wiring makes a short circuit with a voltage supply line, but after an anodic oxidation, it is necessary to etch away the voltage supply and the connection potion of the voltage supply line and the gate wiring and to electrically separate all the gate wirings/electrodes. Therefore, it is necessary to arrange a circuit in consideration of the process margin of etching, which prevents a high-integration design.